In spite of impressive response rates in multiple cancer types, immune checkpoint inhibitors (ICIs) are active in only a minority of patients. Alternative strategies currently aim to combine immunotherapies with conventional agents such as cytotoxic chemotherapies. Here, we performed a study of PD-1 or PDL-1 blockade in combination with reference chemotherapies in four fully immunocompetent mouse models of cancer. We analyzed both the in vivo antitumor response, and the tumor immune infiltrate 4 days after the first treatment. in vivo tumor growth experiments revealed variable responsiveness to ICIs between models. We observed enhanced antitumor effects of the combination of immunotherapy with chemotherapy in the MC38 colon and MB49 bladder models, a lack of response in the 4T1 breast model, and an inhibition of ICIs activity in the MBT-2 bladder model. Flow cytometry analysis of tumor samples showed significant differences in all models between untreated and treated mice. At baseline, all the tumor models studied were predominantly infiltrated with cells harboring an immunosuppressive phenotype. Early alterations of the tumor immune infiltrate after treatment were found to be highly variable. We found that the balance between effector cells and immunosuppressive cells in the tumor microenvironment could be altered with some treatment combinations, but this effect was not always correlated with an impact on in vivo tumor growth. These results show that the combination of cytotoxic chemotherapy with ICIs may result in enhanced, similar or reduced antitumor activity, in a model- and regimen-dependent fashion. The present investigations should help to select appropriate combination regimens for ICIs.

Summary. The cytotoxic activity of cytarabine (ara‐C) in leukaemic blasts depends on activating enzymes such as deoxycytidine kinase (dCK) and inactivating enzymes such as the 5′‐nucleotidases. We have analysed dCK and ‘high‐Km’ 5′‐nucleotidase (cN‐II) mRNA expression by the quantitative real‐time polymerase chain reaction at diagnosis in leukaemic blasts from 115 acute myeloid leukaemia (AML) patients treated with ara‐C. The prognostic value of these parameters as well as that of the cN‐II/dCK ratio was determined. In univariate analyses: (1) low levels of dCK, high levels of cN‐II and a high cN‐II/dCK ratio predicted shorter disease‐free survival (DFS); (2) low levels of dCK and cN‐II/dCK ratio also predicted shorter overall survival (OS). In a multivariate analysis taking into account other clinical and laboratory variables: (1) high cN‐II expression, a high cN‐II/dCK ratio, age ≥ 60 years and an unfavourable karyotype were independent prognostic factors for DFS; and (2) a high cN‐II/dCK ratio, age ≥ 60 years and an unfavourable karyotype predicted shorter OS. Age, karyotype and cN‐II/dCK ratio were used to define a prognostic score that permitted the identification of high‐ and low‐risk groups. Our results suggest that dCK and cN‐II mRNA expression in leukaemic blasts at diagnosis is correlated with clinical outcome and may play a functional role in the resistance to ara‐C in patients with AML.

The benefit of cancer chemotherapy based on alkylating agents is limited because of the action of DNA repair enzymes, which mitigate the damage induced by these agents. The interaction between the proteins ERCC1 and XPF involves two major components of the nucleotide excision repair pathway. Here, novel inhibitors of this interaction were identified by virtual screening based on available structures with use of the National Cancer Institute diversity set and a panel of DrugBank small molecules. Subsequently, experimental validation of the in silico screening was undertaken. Top hits were evaluated on A549 and HCT116 cancer cells. In particular, the compound labeled NSC 130813 [4-[(6-chloro-2-methoxy-9-acridinyl)amino]-2-[(4-methyl-1-piperazinyl)methyl]] was shown to act synergistically with cisplatin and mitomycin C; to increase UVC-mediated cytotoxicity; to modify DNA repair as indicated by the staining of phosphorylated H2AX; and to disrupt interaction between ERCC1 and XPF in cells. In addition, using the Biacore technique, we showed that this compound interacts with the domain of XPF responsible for interaction with ERCC1. This study shows that small molecules targeting the protein-protein interaction of ERCC1 and XPF can be developed to enhance the effects of alkylating agents on cancer cells.